1. Field of the Invention
The present invention generally relates to a system for accommodating radio signal blockage, and in particular to a system for accommodating signal blockage in satellite mobile radio systems.
2. Discussion of the Related Art
With the increasing utilization of satellites for commercial data communication, radio broadcasting via satellite communication is becoming feasible. Radio channels are beamed from earth ground stations to orbiting satellites, which in turn beam the radio channels to individual users all over the earth.
In existing satellite radio systems, a broadcast studio generates analog audio signals much the same as a conventional radio station studio does. For example, an announcer provides real-time narration, and then typically plays music selections from a library of CD music albums. The analog signals are converted to a digital stream of samples, called PCM ("Pulse code modulation"). The conversion is performed for real-time voice or live music performances by passing the analog signals to an A/D ("analog-to-digital converter"). The digital output consists of 16 bit linearly quantized waveform amplitude samples for two channels (stereo right and left), at a sampling rate of approximately 44 kilosamples per second ("ksps"). This is the data stream quality of CD music in temporal sampling resolution and amplitude resolution. In the case of playing music CDS, the A/D step is not necessary since the audio data is already in a digital format on the CD.
The digital audio data is then passed to a satellite ground station for transmission to a satellite on its radio frequency "uplink" carrier. The data can be compressed to save bandwidth and other system resources. The satellite receives the signal from the ground station and retransmits it to the area on the earth's surface where radio reception is desired. For example, the satellite can have a "downlink" beam pattern that covers the continental United States.
The user's receiver (e.g., a car radio) decompresses the digital data, and converts it back to analog signals (one for each stereo channel) with a DAC ("digital-to-analog converter") for subsequent amplification and listening through loudspeakers.
If a user is on a mobile platform, such as a moving automobile, then unique problems are encountered. There are many sources of line-of-sight obscuration as one drives along a typical road or highway. Foliage (trees) attenuates a downlink signal from the satellite, and can even render it unusable if the foliage is particularly dense. Furthermore, there are many sources of truly opaque blockage to the signal, such as telephone poles, billboards, buildings, bridge overpasses, and adjacent large vehicles.
The obstructions can be divided into two groups: (1) Attenuating: Those objects, such as trees or other foliage, which partially reduce signal strength, and (2) Opaque: Those objects which effectively cut off the line-of-sight signal from the satellite completely, such as telephone poles, buildings, overpasses, or even other adjacent vehicles. In the first case, there are several classical means of assuring adequate signal for acceptable reception. These include extra signal strength (called "link margin") broadcast from the satellite to "burn through" moderate foliage coverage, schemes such as interleaving redundant digital bits so that occasional single bit errors have imperceptible effect, and error-detection/correction "coding." As long as there is adequate single (i.e., signal-to-noise ratio), and adequate error correction, radio performance is unaffected.
In the case of totally opaque objects other means have been proposed for accommodation. One is to employ two identical satellites, broadcasting the same radio program material. The satellites are positioned in their orbits such that the angle from each satellite to the user's radio is substantially different. Thus, the changes of both signals being obscured at the same time are unlikely, and radio operation continues without interruption. Another approach is to use two antennas mounted on the car, one fore (on the hood), and one aft (on the trunk lid). Smaller obstructions are thus accommodated, since at least one antenna is in view of a satellite as the vehicle travels along.
However, frequently, the signal from both satellites is lost. During such times, there is total signal loss "blockage" and there is no radio reception. As such, satellite radio use in automobiles is limited to the most barren of terrain, such as the open desert, because the continual and annoying dropout of a radio program is unacceptable to users. Further, use of two satellite systems is costly.
There is, therefore, a need for a satellite radio communication system which enables widespread use and acceptance of satellite radio. There is also a need for such a system to remedy signal loss due to occasional opaque blockages encountered while moving, allowing continuous, uninterrupted radio operation.